Already possible...
The hook is hard-coded to use the 'fa-chevron-circle-up' font from font-awesome, which looks like this:
https://fontawesome.com/v4.7.0/icon/chevron-circle-up
Changing the color via CSS, which is currently possible, will change the font's colour...but people perceive it as the background changing; the up arrow is not going to change.
However...you can also change the font used via CSS, by doing the following instead:
#backtoTop .fa-chevron-circle-up::before{
content:"\f077"; /* changes the fa-chevron-circle-up unicode value (f138) to use the unicode value for fa-chevron-up (f077) instead */
color:green; /* Your chosen colour */
}